The present invention relates to organic electroluminescent (EL) devices, also known as organic light-emitting diodes (OLEDs), and particularly to transferring organic material from a donor to form one or more organic layers in such devices.
In color or full-color organic electroluminescent (EL) displays having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media is required to produce the RGB pixels. The basic EL device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers or regions within a layer is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the light-emitting layer of the organic EL medium. Other organic layers present in the organic EL medium commonly facilitate electronic transportation, and are referred to as either the hole-transporting layer (for hole conduction) or electron-transporting layer (for electron conduction). In forming the RGB pixels in a full-color organic EL display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in commonly-assigned U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. It has been difficult to achieve high resolution of pixel sizes using shadow masking. Moreover, there are problems of alignment between the substrate and the shadow mask, and care must be taken that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is difficult to manipulate the shadow mask to form appropriately positioned pixels. A further disadvantage of the shadow mask method is that the mask holes can become plugged with time. Plugged holes on the mask lead to the undesirable result of non-functioning pixels on the EL display.
There are further problems with the shadow mask method, which become especially apparent when making EL devices with dimensions of more than a few inches on a side. It is extremely difficult to manufacture larger shadow masks with the required precision (hole position of xc2x15 micrometers) for accurately forming EL devices.
A method for patterning high-resolution organic EL displays has been disclosed in commonly-assigned U.S. Pat. No. 5,851,709 by Grande et al. This method is comprised of the following sequences of steps: 1) providing a donor substrate having opposing first and second surfaces; 2) forming a light-transmissive, heat-insulating layer over the first surface of the donor substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the donor substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable, color-forming, organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. This creates many of the same problems as the shadow mask method, including the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Using an unpatterned donor sheet and a precision light source, such as a laser, can remove some of the difficulties seen with a patterned donor. Such a method is disclosed by Littman in commonly-assigned U.S. Pat. No. 5,688,551, and in a series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553).
In commonly-assigned U.S. Pat. No. 5,937,272, Tang has taught a method of patterning multicolor pixels (e.g. red, green, and blue subpixels) onto a thin-film-transistor (TFT) array substrate by vapor deposition of an EL material. Such EL material can be precoated on one surface of a donor support material and transferred to a substrate by vapor deposition in a selected pattern (as in FIGS. 4, 5, and 6 in the aforementioned U.S. Pat. No. 5,937,272).
The EL material transfer is preferably done in a vacuum chamber such as Tang describes in the aforementioned patent and, in particular, vacuum is preferably maintained between the donor and substrate. The donor and substrate must also be kept in close proximity during the EL transfer (less than 250 micrometers between the coating and raised portions of the substrate as taught by Tang). Furthermore, the donor can be in contact with the raised portions of the substrate and thereby maintain sufficient spacing between the coating and the recessed portions of the substrate where the EL material is deposited. In any case, a method of holding the donor and substrate in contact in a vacuum chamber while maintaining vacuum between the donor and substrate is required.
Isberg, et al., in commonly assigned European Patent Application 1 028 001 A1, have disclosed the additional use of an adhesion-promoting layer between the donor layer and substrate. While this would help promote the close contact required by Tang, it would be disadvantageous because the adhesion-promoting layer can introduce impurities in the form of the adhesive.
It is therefore an object of the present invention to provide a more effective way of positioning a donor element relative to an OLED substrate for facilitating the formation of one or more layers of organic material.
This object is achieved by an apparatus for transferring organic material from a donor onto a substrate to form a layer of organic material on one or more OLED devices, comprising:
(a) a vacuum chamber under reduced pressure;
(b) a first fixture disposed in the vacuum chamber;
(c) a second fixture disposed in the vacuum chamber and to be aligned with and engaging the first fixture to clamp the donor and substrate which are supported by at least one of the first and second fixtures and forming a second chamber positioned relative to a non-transfer surface of the donor;
(d) means for supplying a gas to the second chamber to apply pressure to the non-transfer surface of the donor so as to ensure the position of the donor relative to the substrate;
(e) the first fixture including a movable member movable between a closed position defining a surface of the chamber and an open radiation-receiving position to permit transmission of radiation to the non-transfer surface of the donor so that heat will be produced and the organic material will transfer from the donor to the substrate; and
(f) an illumination source for directing radiation onto the donor through the open radiation-receiving position to cause the transfer of organic material to the substrate.
This object is also achieved by a method for transferring organic material from a donor onto a substrate to form a layer of organic material on one or more OLED devices, comprising the steps of:
(a) aligning first and second fixtures in a reduced pressure environment and positioning the substrate and donor in a chamber defined by the aligned first and second fixtures;.
(b) increasing the pressure applied to the non-transfer surface of the donor so as to ensure the position of the donor relative to the substrate;
(c) after step b, moving a member provided on the first fixture from a closed to an open position which permits the transmission of radiation onto the non-transfer surface of the donor so that heat will be produced and organic material transferred from the donor to the substrate; and
(d) illuminating with radiation the donor through the open radiation-receiving position to cause the transfer of organic material to the substrate.
An advantage to this method is that it provides for maintaining a uniform spacing between a donor material and a substrate in an ambient vacuum or vacuum environment and where it is further preferred that vacuum be maintained between the donor and substrate. This enables suitable clamping in an environment (vacuum) that is advantageous for lowering contamination. A further advantage is that the method and apparatus reduce the likelihood of optical distortion. A further advantage is that the apparatus described herein can be more easily scaled up to produce larger display units than other methods. A further advantage is that the chamber formed by the first and second fixtures in the apparatus can be made small so that it can be pressurized and evacuated quickly, allowing high throughput. A further advantage is that this invention can be filly automated including donor and substrate media handling. The present invention is particularly suitable for forming organic layers over a large area having a number of OLED display devices, which are in the process of being formed, thereby increasing throughput.